1. Technical Field
This disclosure relates to optical imaging systems and, more particularly, to a method and apparatus for correcting for color and/or intensity vignetting in an image and in the imaging system which generated the image.
2. Discussion of the Related Art
Digital imaging systems typically include an optical system for gathering and projecting or imaging light onto a receiver, detector or sensor. The optical system typically includes one or more imaging lenses for forming the image on the sensor. The sensor is commonly a two-dimensional array of optical detectors or sensors, such as complementary metal-oxide-semiconductor (CMOS) image sensors.
Vignetting is a common phenomenon which occurs in optical systems. It refers to the color and intensity variation with respect to the lens' chief ray angle. As the chief ray angle increases, the quantum efficiency of the sensors or detectors decreases, resulting in a decrease in the color and intensity as the distance from the center of the image increases.
Many approaches to correcting for the vignetting artifact have been introduced. Most correction methods require specific scenes for calibration, which usually must be uniformly lit, and the calibrated model is applied to the real image or video. In practice, the vignetting model actually changes with light spectrum and focal length, which is difficult to calibrate across different light sources. This phenomenon is more pronounced in CMOS image sensors with small pixel size. Other approaches work on a single image individually and require image segmentation and a large buffer, which is too complex to embed in an image sensor.